os.environ is not inherited on Windows

[timfel]

Simple test:

>>> import os, subprocess, sys
>>> os.environ["FOOBAR"] = "1"
>>> subprocess.run([sys.executable, "-c", "import os; print(os.environ['FOOBAR'])"])

This should print "1", but errors instead

[timfel]

This is blocking removal of the virtualenv workaround in cibuildwheels: pypa/cibuildwheel#2501